Field of the Invention
The field of invention relates to the production of hydrocarbons. More specifically, the field relates to a composition and method for stimulating the production of hydrocarbons.
Description of the Related Art
The production of hydrocarbons can be stimulated by removing filter cake from the well bore wall of a well bore and the face of the hydrocarbon-bearing formation, and by forming wormholes into the hydrocarbon-bearing formation. Such fluid pathways allow hydrocarbons to traverse the hydrocarbon-bearing formation, and the wormholes can access previously inaccessible, fluidly isolated portions the hydrocarbon-bearing formation. Matrix acids, including hydrofluoric acid and hydrochloric acid, are well known in the art to achieve such goals. The acids react with the portions of the filter cake and the hydrocarbon-bearing formation susceptible to acid attack (for example, silicates and carbonates) upon contact.
One problem with using a strong acid in a hydrocarbon-bearing formation is that the filter cake removal may be non-uniform due to differences in the composition of the filter cake, leaving the face of the hydrocarbon-bearing formation with blockages. This reduces the total volumetric flow of fluid that the hydrocarbon-bearing formation is capable of conveying through the face. The other issue is that strong matrix acids tend to react with the formation in an instantaneous manner upon contact due to the amount of acid-reactive material present. This results in the depletion of the strong acid species almost immediately upon contact with the formation. It may also result in the creation of wide, shallow (that is, not penetrating deep into the formation relative to the distance from the well bore) and non-uniform wormholes through the hydrocarbon-bearing formation. These shallow wormholes are not the desirable deep fluid pathways through the hydrocarbon-bearing formation that act as tributaries for the well bore and enhance the rate and efficiency of hydrocarbon extraction.
Esters of acids have been used to overcome the problems of strong matrix acids reacting too quickly and unevenly with both the filter cake and the hydrocarbon-bearing formation. Esters of acids can hydrolyze in the presence of water into organic acids to form acidic solutions. As an acidic solution forms, the resulting acid is consumed, which drives the reversible hydrolysis reaction to completion. The hydrolysis rate is determined by the type of ester and the temperature of the aqueous solution. It is known that an ester of acetic acid in water at 100° C. may completely hydrolyze into acetic acid within 24 hours.
However, even this amount of time may not be sufficient to deliver the ester of the acid to the treatment site. Well bores in modern practice are extended horizontally for distances up to and beyond 30 kilometers from a single vertical well. In addition, modern well configurations may include 1) multi-lateral wells, which have multiple branches either from a single vertical or a horizontal shaft, or both; and 2) multi-tier wells, where different hydrocarbon-producing formations are tapped using a single production well. Navigating and traversing through complex well bores to a production face may therefore take several days.
It is desirable to develop a method of removing filter cake more uniformly or acidizing a hydrocarbon-bearing formation more deeply, or both, using a system that allows the user adequate time to maneuver the system in place for targeted application and distribution.